1. Field of the Invention
This invention relates to a low-capacitance package suitable for optoelectronic devices for optical communication or optical measurement and to an optoelectronic device mounted on the package, for example, a photodiode (PD) module.
2. Description of Related Art
This application claims the priority of Japanese Patent Application NO. 8-219218 (219218/96) filed Jul. 31, 1996 which is incorporated herein by reference. The following explanation about various packages for photodiodes will provide a background for understanding the present invention.
[(1) Prior Art of Packages]
(Type 1 of prior packages) Metal-base package
FIG. 1(a) and FIG. 1(b) show a header of a known package called TO46 which has conventionally been used for photodiodes (PDs). FIG. 1(a) is a plan view, FIG. 1(b) is a sectional view of the header part of a TO46 package. An eyelet (1) of a disk shape has three lead pins (2), (3) and (4). The disc part is called an eyelet, because two pins of the disc seem to be eyes in the plan view. The leads are made from a FeNiCo alloy. One of the three is a case pin (2). The others are fixed in vertical holes dug in the eyelet (1) by a low-melting point glass (5). The glass insulates the lead pins from the eyelet (1). The disc eyelet (1) has a circular flange at the bottom and a base at the top. The flange has a diameter of 5.4 mm .phi.. The base diameter is 4.22 mm .phi.. The metal eyelet (1) has a thickness of 1.12 mm. A photodiode device is produced by die-bonding a photodiode chip directly on the central part of the eyelet (1), connecting electrodes of the chip to the lead pins (3) and (4), covering the eyelet (1) by a cap having a flat glass window or a lens, and sealing the inner space with an inert gas. The eyelet (1) is made from soft steel. This is a metal-base type example of prior packages.
(Type 2 of prior packages) Glass-base package
FIG. 2(a) and FIG. 2(b) show another known package called TO18. The package has a hat-shaped eyelet (6) made by bending a thin metal plate in a convex shape. The hollow of the eyelet (6) is filled with glass (7). The package having such an eyelet is called a glass-base type package. Three lead pins (8), (9) and (10) are furnished to the eyelet (6). One pin (8) is a case pin. Two pins are insulated by the glass (7) from the metal eyelet (6). The glass (7) supports the pins at the bottom of the metal eyelet (6). The metal plate has a thickness of 0.21 mm. The eyelet has a metal disc part of a 4.2 mm diameter and a bottom flange of a 5.4 mm diameter. The spacing between the neighboring pins is 2.54 mm in the eyelet (6). A photodiode device is built up by die-bonding a photodiode chip on the top of the eyelet (6), wirebonding electrodes of the chip to the lead pins, fitting a cap having a window or a lens, and sealing the inner space of the cap with an inert gas.
(Type 3 of prior packages) CD-laser package
A third type of package, known as the CD-laser package is inherently not a package for a photodiode but rather is a package for a laser diode. Laser diode packages are sometimes used as photodiode (PD) packages. Both a laser diode and a photodiode are optoelectronic devices. A laser diode actually produces light when electrically driven. A photodiode is a passive device for sensing light. Its characteristics change based on light incident thereon. FIG. 3 is a perspective view of a package of a compact disc (CD) laser diode. This is a 5.6 mm .phi. CD laser package. An eyelet (11) is a metal disc having a diameter of 5.6 mm. The 5.6 mm .phi. disc has three lead pins (12), (13) and (14). A laser diode chip (15) is vertically fixed on a side of a pole (16) of the eyelet (11). A photodiode chip (19) for monitoring the LD power is mounted on the central portion of the eyelet (11). A cap (17) having a window (18) is fitted on the flat surface of the eyelet (11). Since the diameter of the foot of the cap (17) is smaller than the diameter of the eyelet, the cap (17) can be positioned at an arbitrary spot on the eyelet. The cap (17) covers and protects the chips. The inner space is filled with an inert gas.
This type of package was originally intended to house a laser diode. It has advantages of a large thickness of the eyelet, high heat diffusivity, high freedom of positioning a cap on the eyelet, since the cap can be fixed at an arbitrary position on the flat eyelet by e.g., electric-resistance welding. The high heat conduction results from the thick eyelet. Coaxial alignment of the PD chip, the LD chip and the cap facilitates the design of optical system of the device. Conveniently, the optical axis adjustment is simplified to an axial direction and a radial direction. These advantages make the metal header suitable for use as an LD package.
The eyelet has the pole (16) for supporting the laser (15) vertically. The existence of the pole (16) is a characteristic as an LD package. If the pole (16) is removed from the eyelet (11), the eyelet (11) becomes flat. The flat eyelet can be diverted as an eyelet of a PD package. If the eyelet without the pole is used as an eyelet of a PD, the packages of PDs become similar to the packages of LDs. The use of common packages can treat both PDs and LDs by the same jigs in an assembly line and in an inspection line. CD (compact disc) lasers are widely used in many types of applications. Mass production of CD lasers has decreased the cost of packages of CD lasers. Low-cost is another advantage of the use of LD packages as PD packages.
FIG. 4 is a sectional view of a photodiode device mounted on a thick, metal eyelet diverted from a CD laser package. An eyelet (20) is provided with three lead pins (21), (22) and (23). One is a case pin (22) directly fixed to the eyelet (20) but the other pins (21) and (23) are insulated from the case (eyelet) (20). The pins (21) and (23) are fixed in holes (24) of the eyelet (20) by insulating, sealing glass (25). A submount (26) is fixed at the center of the eyelet (20). A photodiode (PD) chip (27) is mounted on the submount (26). A cap (28) is welded at the bottom to the eyelet (20). The cap (28) has a top opening and a lens (29) on the opening. Light beams emitted from an end of an optical fiber are converged to the PD chip (27) by the lens (29). The cap (28) seals the inner space of the package with an inert gas. Since the PD (2) must be insulated from the case (eyelet) (20), the PD chip (27) is mounted via an insulator, i.e., the submount (26) upon the package.
[(2) Prior PD modules]
(Prior art 4: photodiode module)
A conventional photodiode module will now be explained. FIG. 5 shows a photodiode module having a package and a PD chip furnished in the package. The package is the same as the package shown by FIG. 4 which has been explained as a PD package diverted from prevailing packages for LDs of FIG. 3. The photodiode chip (27) is soldered on the submount (26) fixed on the eyelet (20). The cap (28) is fixed on the package (20) and the inner room is filled with an inactive gas.
A cylindrical sleeve (30) is fitted around the outside of the eyelet (20). A cylindrical ferrule holder (31) with an axial opening is welded at an optimum position on the sleeve (30). A ferrule (33) with a narrow vertical hole is inserted in the opening of the ferrule holder (31). An end of a single-mode fiber (34) has been inserted into the vertical hole of the ferrule (33). The front end of the ferrule (33) has been polished in order to prevent the reflected light from returning to the LD. An elastic, conical bend-limiter (35) is capped on the tail of the holder (31) for prohibiting the optical fiber (34) from bending in an excessive small radius of curvature. An anode (annular p-electrode) of the PD chip (27) is wirebonded to the anode pin (23). A cathode (bottom n-electrode) of the PD (27) is soldered on the metallized film on the submount (26). The cathode is connected to the cathode pin (21) by wirebonding the metallized film to the pin (21). In this example, the cathode must be insulated from the case (20) by the submount (26). The following amplification circuit requests the insulation of the cathode from the case (20). Prior packages and prior modules have been clarified. A photodiode is a device of converting light power to a photocurrent. A photodiode requires an electric circuit for biasing the pn-junction, converting the photocurrent to a voltage and amplifying the photocurrent. Then typical electric circuits for operating photodiodes are explained now.
[(3) Prior art of the electric circuits for driving photodiodes]
(Prior art 5: Resistance Load Circuit)
FIG. 6 shows a well-known photodiode circuit having a resistance load. The load resistance R.sub.L is joined to the anode of a photodiode (PD). The cathode of the PD is connected to the power source voltage Vb. The other end of the R.sub.L is grounded. Thus the pn-junction of the PD is reversely biased by Vb. The case pin is grounded. The cathode should be insulated from the case. In many cases, the load resistance is 50 .OMEGA.. The photocurrent I.sub.p is converted to a voltage by the resistor R.sub.L. An amplification device amplifies R.sub.L I.sub.P by a certain ratio to an output voltage. This circuit excels in the speed of response, because the load resistance is low enough. This type circuit can operate on signals up to several gigahertzs (GHz). However, this circuit has the drawbacks of low sensitivity due to the low load resistance and low signal/noise (S/N) ratio due to poor sensitivity. This circuit is suitable only for strong input signals. Weaker input signals require circuits that have a higher input impedance.
(Prior Art 6: Trans-impedance Circuit)
FIG. 7 shows a trans-impedance circuit for a PD. The anode of the PD is connected to the input of an amplifier(AMP). The output of the AMP is fed back to the input by a resistor Rf. In this case, Rf essentially constitutes the input impedance. The input impedance can be made sufficiently large by increasing the value of resistance Rf. A high input impedance can strengthen the signals and enhance the S/N ratio. This circuit is suitably used for the detection of digital signals (binary signals). In the case of digital signals, even if the input signal contains a signal distortion, the binary signals can be reformed by inserting a wave-trimming circuit. This circuit is effective when the signals do not require a high speed processing and the signals are faint enough.
(Prior Art 7: Transformer Load Circuit)
FIG. 8 shows a transformer load circuit. The anode of a photodiode (PD) is connected to one end of a primary coil of a transformer. The other end of the primary coil is connected to an end of a second coil. The other end of the second coil is grounded. Both coils are air gap coils. The primary coil is coupled with the secondary coil by a magnetic field. The turn ratio of the coils is N:1 where N is larger than 1 (N&gt;1). The input impedance can be enhanced by raising the turn ratio. The ratio of impedance is in proportion to N.sup.2 :1. For example, if the turn ratio is 2:1 and the impedance of the AMP is 75 .OMEGA., the input impedance on the PD is 300 .OMEGA.. The current ratio is raised to 1:N. Thus the current on the amplifier (AMP) is N times as big as the photocurrent of the PD. Thus the load transformer effectively raises the input impedance and amplifying the current.
Further, the AMP amplifies the voltage of the intermediate terminal of the transformer. The circuits of FIG. 6 and FIG. 7 cannot regenerate analog signals with high fidelity since the resistance R.sub.L and the AMP induce noise. In contrast, this circuit has an advantage of low noise since the PD photocurrent is directly amplified by the transformer. The transformer-load circuit is suitable for analog signals. Fiber-Optic-CATV systems which send analog signals by light, in general, employ such transformer load circuits. However, this circuit suffers from a drawback of narrowing the frequency range because of the current-amplification by a transformer.
The weak point of the transformer load circuit is the reduction of the frequency range width due to the coil coupling. The reason why the transformer coupling reduces the frequency range is explained by referring to FIG. 9. FIG. 9 exhibits the relation between the amplifier output (dB) and the frequency (MHz) when the turn ratio is 2:1, the impedance of the AMP is 75 .OMEGA. and the input impedance is 300 .OMEGA.. The abscissa is the frequency (MHz) of the input signals. The ordinate is the logarithmic power of the AMP. The signal power at 50 MHz is deemed to be a standard, and is set to be 0 dB. The output power must exist within the scope of .+-.1 dB from the standard. In this case, the curve crosses the horizontal line of -0.1 dB down at 600 MHz. Thus the circuit can amplify the signals up to 600 MHz. The upper limit is 600MHz. The frequency range width is 600 MHz in the example. The transformer coupling decreases the frequency range width. 600 MHz was sufficient for prior CATV systems, but future CATV systems will require a wider frequency range.
Initially, optical CATV systems required only a few channels for transmission of signals. However, the channel requirement has increased. Present CATV systems require 40 channels as a standard. 40 channels require a wide frequency range of about 400 MHz to 450 MHz. Recent developments will cause CATV systems to require 80 channels to 110 channels. Such an increase in the number of channels will require a further widening of the frequency range to 860 MHz instead of 450 MHz. Namely, the frequency range must be doubled for future pervasion of the Fiber-Optic-CATV. Conventional PD modules such as shown in FIG. 9 have only a 600 MHz frequency range, if the restriction of .+-.1 dB is imposed to the performance of PD modules. Future development of the Fiber-Optic-CATV needs photodiode modules having a still wider frequency range. The expansion of frequency range of PD modules is not all of the requirements in the future Fiber-Optic-CATV system. There are problems for PDs other than the frequency range.
Low price is one of the most important requirements for further prevalence of the Fiber-Optic-CATV. It is desirable to make a new transformer-load type PD module having a wider frequency range without enhancing cost and impairing gain. PD modules must have a frequency range of at least 860 MHz for receiving signals of about 100 channels.
What restricts the frequency range of PDs? The properties of the photodiode themselves, of course, have an influence on the frequency range of the PD module. A more serious factor that restricts the frequency range of PD modules is the electrostatic capacitance C between a PD chip and a package. The frequency range of PDs is restricted by the time constant .tau. (=CR) which is a product of the chip.cndot.package capacitance C and the resistance R of a load resistor.
The frequency range can be expanded by lowering the load resistance R or decreasing the capacitance C between a chip and a package.
This invention aims at expanding the frequency range by reducing the electrostatic capacitance C. The mounting of a PD chip upon a packages has been explained hitherto. The capacitance varies according to the size of a chip, the size of a package, the diameter of the pin hole and the length and diameter of the pins. The other aim is related to a new way of mounting the chip on the package. The minimum electrostatic capacitance is determined by the mounting structure of a chip on a package. The capacitances between each pin and a case are explained by citing several kinds of packages hereafter.
(1) TO46 package (FIG. 1)
An example of FIG. 1 has a flange of 5.4 mm .phi., a base of a 4.22 mm .phi.and a thickness of 1.12 mm. The circle on which the pins exist has a diameter of 2.54 mm. The eyelet is made from mild steel. The case (eyelet) has three pins, i.e. a case pin, an anode pin and a cathode pin. Since the anode pin and the cathode pin are insulated from the case, some capacitances occur between each pin and the case. The capacitances are measured in the package.
Case.cndot.anode capacitance=0.62 pF .about.0.67 pF
Case.cndot.cathode capacitance=0.8 pF
Here, the case.cndot.cathode capacitance is measured on condition that the chip is bonded on a submount of 1.0 mm square .times.0.25 mm being fitted on the case. Thus the submount contributes to the cathode.cndot.case capacitance. Thus the sum of the capacitances is about 1.45 pF.
(2) TO18 package (FIG. 2)
The FIG. 2 arrangement has a flange of 5.4 mm .phi. and a base of 4.2 mm .phi.. The circle on which the pins align has a diameter of 2.54 mm .phi..
Case.cndot.anode capacitance=0.42 pF.about.0.47 pF
Case.cndot.cathode capacitance=0.42 pF.about.0.47 pF
Submount capacitance=0.27 pF
The total of capacitances is approximately 1.3 pF.
It is inevitable that the frequency range is suppressed by the package capacitance as long as a PD module uses such a large capacitance package. Although it is well known that the package capacitance restricts the frequency range of a PD, people believe that current packages have already have the minimum capacitance that can be achieved and that there is no room for reducing the package capacitance any more. The Inventors, however, have noticed the possibility of reducing package capacitance further by providing a new package structure.